1. Field of the Invention
This invention relates to an electro-luminescence display, and more particularly to an electro-luminescence display device and a method of driving the same wherein a brightness of full white is controlled depending upon a brightness of external environment, thereby controlling a brightness mode.
2. Discussion of the Related Art
An electro-luminescence (EL) display is a self-luminous device in which a phosphorous material emits light by recombination of electrons and holes. The EL display is largely classified into an inorganic EL display device and an organic EL display device, depending upon its material and structure. The EL display has the same advantage as cathode ray tubes (CRT) in that it has a faster response speed than passive-type light-emitting devices such as liquid crystal displays (LCD), which require a separate light source.
FIG. 1 is a sectional view illustrating a general structure of an organic EL device for explaining a light-emitting principle of the EL display device. Referring to FIG. 1, an organic EL display device includes an electron injection layer 4, an electron carrier layer 6, a light-emitting layer 8, a hole carrier layer 10 and a hole injection layer 12 that are sequentially disposed between a cathode 2 and an anode 14. When a voltage is applied between a transparent electrode, that is, the anode 14 and a metal electrode, that is, the cathode 2, then electrons produced from the cathode 2 are injected, via the electron injection layer 4 and the electron carrier layer 6, into the light-emitting layer 8, while holes produced from the anode 14 are injected, via the hole injection layer 12 and the hole carrier layer 10, into the light-emitting layer 8. Thus, the electrons and the holes fed from the electron carrier layer 6 and the hole carrier layer 10, respectively, are collided and recombined at the light-emitting layer 8 to generate light. Then, this light is emitted, via the transparent electrode (i.e., the anode 14), into the exterior to thereby display a picture.
Such an EL display device employs either a surface-area divisional driving method or a time divisional driving method to express gray levels. The surface-area divisional driving method expresses a gray level by dividing one pixel into a plurality of sub-pixels, each of which is independently driven in accordance with a digital data signal. However, such a surface-area divisional driving method has a problem in that the pixel structure becomes complicated. On the other hand, the time divisional driving method expresses a gray level by controlling a light-emission time of pixels. In other words, it divides one frame into a plurality of sub-frames to display a gray level, and each sub-frame interval is further divided into an emission time and a non-emission time. Thus, a gray level of a pixel is expressed by summing the emission time of each sub-frame within one frame interval. Because EL display devices have a faster response speed than LCD devices, the time divisional driving method is generally employed.
FIG. 2 illustrates a time divisional driving method employed to drive an EL display device according to a related art. Referring to FIG. 2, the time divisional driving method divides each frame into a plurality of sub-frames SF corresponding to each bit of a digital video signal for gray level expression. In FIG. 2, a 12-bit digital data signal is used to express 256 gray levels, and one frame is divided into 12 sub-frames SF1 to SF12 in such a manner to correspond to the 12-bit digital data signal. The first sub-frame SF1 of the 12 sub-frames SF1 to SF12 corresponds to the least significant bit of the digital data signal, while the 12th sub-frame SF12 thereof corresponds to the most significant bit of the digital data signal.
Each of the 12 sub-frames SF1 to SF12 is divided into an emission time of LT1 to LT12 and a non-emission time of UT1 to UT12. In this case, the emission time LT1 to LT12 of each sub-frame SF1 to SF12 can use either a binary code having a ratio of 1:2:4:8:16:32: . . . or a non-binary code such as 1:2:4:6:10:14:19: . . . for expressing 28 (i.e., 256) gray levels using a 12-bit digital data signal.
During each sub-frame (SF1 to SF12) interval, the EL display device sequentially scans the entire pixels in a vertical direction, that is, in a direction from the upper portion of the EL panel to the lower portion thereof for its light-emission. Thus, the emission time LT1 to LT12 of each sub-frame (SF1 to SF12) interval follows the oblique line shown in FIG. 2 within each sub-frame SF1 to SF12. All the emission times within each sub-frame SF1 to SF12 are summed during one frame interval to thereby express a gray level of a desired picture.
Because such a time divisional driving method according to the related art expresses a desired gray level by summing the emission time LT1 to LT12 of each sub-frame SF1 to SF12 during one frame interval, a full white brightness of the EL display device is fixed, when displaying pictures, irrespective of where the EL display is, that is, a brightness of the external environment. Therefore, the related art EL display device driven by the time divisional driving method has a problem of large power consumption.